1. Field of the Invention
The invention relates to a method and apparatus useful in determining subsurface pressure in a well. More particularly, the invention relates to such a method and apparatus useful in conjunction with making transient pressure determinations.
2. Description of the Prior Art
Subterranean pressure data from wells is useful for a wide variety of purposes including: determining the efficiency of the well completion; establishing the need for and success of a well stimulation treatment; determining the general type of well treatment desirable; establishing whether or not a well is connected to other wells; and the like. One type of pressure which can provide valuable information about a reservoir is the transient pressure response of a well, i.e., the pressure response which results from a change in a well's production rate or injection rate. A transient pressure in a well can be created by putting a closed-in well on production or injection or by changing the flow rate of a well which has been producing or into which fluid has been injected at a constant flow rate for some period of time and has reached a pseudo steady-state behavior. Types of transient pressure behavior include: pressure drawdown and buildup in producing wells; skin damage tests; pressure falloff in injection wells; multiple-rate tests in both producing wells and injection wells; and interference or pulse tests between two or more wells in a reservoir.
Several types of transient pressure tests depend on determining the tubing pressure at a subterranean location immediately after a tubing valve at the surface of a well have been closed. Such tests are difficult to run and control because fluid has been flowing in the tubing immediately prior to the shut-in. A change in the flow rate of the well by closing a valve at the surface can result in an inaccurate indication of the subsurface pressure as measured by a subsurface pressure gauge. Because of the large volumes of the well tubing and the tubing-casing annulus, closing only a surface valve does not instantaneously stop flow into or out of the wellbore at the bottom of the well. Interpretation of these transient pressure tests is based on the assumption that no flow occurs after the surface tubing valve is closed.
Various techniques of mathematical manipulation have been proposed to minimize the error from the continued flow of fluid, usually called "afterflow," occurring after a surface valve is closed. However, these techniques are subject to interpretation and errors. Various mechanical methods have previously been proposed for closing off the bottom of the well tubing to fluid flow at the same time that a surface valve is closed. Such device heretofore have been expensive and complicated. Some of these methods involve running a pack-off apparatus as soon as the surface valve is closed. Obviously there is a considerable time delay required to run the apparatus to the desired downhole location and set it. An electric packer run on a conductor cable has been used but requires several minutes to set resulting in loss of valuable data, as the delay in shutoff results in continued flow through the tubing. A tubing packer gauge hanger device requiring wireline jars to set is has been unsatisfactory because gauge damage often results from the jarring action.
It is an object of this invention to provide a method and apparatus for running transient pressure tests.
It is a further object to provide such a method and apparatus which shuts off fluid flow into or out of the bottom of the well tubing almost instantaneously following closing of a tubing valve at the surface.
It is a still further object to provide such an apparatus which may be quickly opened and retrived following completion of the subsurface pressure determination to allow flow through the tubing to resume.
It is another object to provide such an apparatus, the opening and closing of which may be controlled from the surface of the well.
It is still another object to provide a method and apparatus which will maintain shut-in at the bottom of well tubing of an observation well during an interference test.
Other objects, advantages, and features of the invention will be apparent from the following discussion, drawings, and appended claims.